The present invention relates to a process for producing phthalaldehyde (i.e. o-phthalaldehyde). Phthalaldehyde is a compound widely used as a raw material or intermediate for agricultural chemicals, pharmaceutical drugs and cosmetics and is specifically important as a raw material for insecticides for industrial use.
Phthalaldehyde has been produced by, for example, a process of reducing corresponding phthaloyl dichloride, a process of directly oxidizing a methyl group in a side chain of o-xylene, or a process of hydrolyzing xcex1,xcex1,xcex1xe2x80x2,xcex1xe2x80x2-tetrachloroxylene. However, these processes each have various disadvantages and are not industrially or commercially satisfactory.
For example, the process of reducing phthaloyl dichloride requires an expensive raw material as well as an expensive palladium compound as a catalyst and exhibits a low yield. The process of directly oxidizing a methyl group in a side chain of o-xylylene is performed by a liquid phase process or a gas phase process, each of which cannot exhibit a sufficient yield. The process of hydrolyzing xcex1,xcex1,xcex1xe2x80x2,xcex1xe2x80x2-tetrachloroxylene requires hardly available xcex1,xcex1,xcex1xe2x80x2,xcex1xe2x80x2-tetrachloroxylene as a raw material and often yields by-products in the hydrolysis procedure.
Accordingly, an object of the present invention is to provide a process for efficiently producing phthalaldehyde from easily available raw materials.
After intensive investigations to achieve the above object, the present inventors have found that, by allowing easily available o-xylylene glycol or o-xylylene oxide to react with nitric acid, phthalaldehyde can be produced in a high yield and thereby high-purity phthalaldehyde can be obtained at low cost without using a hardly handleable chemical substance such as a halogen derivative as a raw material and/or a metallic catalyst. The present invention has been accomplished based on these findings.
Specifically, the present invention provides a process for producing phthalaldehyde, comprising the step of allowing at least one compound selected from o-xylylene glycol and o-xylylene oxide to react with nitric acid to thereby yield phthalaldehyde. In this process, the amount of nitric acid is about 0.2 to about 20 moles per mole of the at least one compound selected from o-xylylene glycol and o-xylylene oxide.
[o-Xylylene Glycol and o-Xylylene Oxide]
At least one compound selected from o-xylylene glycol and o-xylylene oxide is used as a reaction component (substrate) in the present invention. The benzene ring of o-xylylene glycol and o-xylylene oxide may further have one to four of other substituents. Such substituents include, for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, t-butyl, and other alkyl groups, of which C1-C4 alkyl groups are preferred; fluorine, chlorine, bromine, and other halogen atoms; methoxy, ethoxy, isopropoxy, and other alkoxy groups, of which C1-C4 alkoxy groups are preferred; methoxycarbonyl, ethoxycarbonyl, and other alkoxycarbonyl groups, of which C1-C4 alkoxy-carbonyl groups are preferred; acetyl group, benzoyl group, and other acyl groups; acetoxy group, and other acyloxy groups; hydroxyl group; carboxyl group; nitro group; unsubstituted, mono-substituted, or di-substituted amino groups; nitro group; and cyano group.
Each of o-xylylene glycol and o-xylylene oxide can be used alone or in combination. When they are used in combination as a mixture, the mixing ratio thereof is not specifically limited.
[Nitric Acid]
Nitric acid is used as an oxidizing agent in the present invention. Such nitric acid is not specifically limited, and commercially available one can be used. Nitric acid can be diluted with water or an inert organic solvent before use. The amount of nitric acid is, for example, from about 0.2 to about 20 moles, and preferably from about 1 to about 10 moles per mole of the substrate.
The process of the present invention does not always require a catalyst, but a catalyst can be used, for example, in order to shorten a reaction time. The catalyst includes, for example, compounds each containing any of metallic elements having an atomic number of 23 to 29 (V, Cr, Mn, Fe, Co, Ni, and Cu). Examples of such metallic compounds are, of the metals, nitrates, chlorides, sulfates, carbonates, and other salts; and oxides. Preferred catalysts include, for example, vanadium compounds. The amount of the catalyst can be appropriately set depending on, for example, the type of the catalyst and the concentration of nitric acid and is generally from about 0.002 to about 0.1 part by weight relative to 1 part by weight of the substrate. An initiator such as sodium nitrite, nitrogen dioxide, or benzoyl peroxide and/or azobisisobutyronitrile known as a radical initiator can be used in the reaction.
[Reaction]
The reaction is performed in the presence of, or in the absence of, a solvent. Such solvents include, for example, water; acetic acid, other organic acids, hydrochloric acid, perchloric acid, sulfuric acid, other inorganic acids, and other acids; dioxane, and other ethers; and mixtures of these solvents. A reaction rate often increases when the medium has a higher acidity. Preferred solvents include a water-acetic acid mixture, and other mixtures of water and an acid. In this case, the reaction rate can be controlled by regulating the ratio of water to the acid. When a solvent mixture of water and acetic acid is used, the ratio of water to acetic acid in a reaction system is, for example, such that the former/the latter is from about 0/100 to about 99/1.
A reaction temperature can be appropriately selected depending on, for example, the type of the substrate and is, for example, from about 20xc2x0 C. to about 200xc2x0 C., preferably from about 50xc2x0 C. to about 150xc2x0 C., and more preferably from about 65xc2x0 C. to about 120xc2x0 C. The reaction can be performed at normal atmospheric pressure or under a pressure (under a load). The reaction time varies depending on, for example, the reaction temperature and is generally from about 0.1 to about 12 hours, and preferably from about 0.2 to about 8 hours. The reaction can be performed according to any system such as a batch system, semi-batch system or continuous system.
According to the process of the present invention, the reaction can selectively and efficiently oxidize a methylene carbon atom at the alpha position of the benzene ring of the substrate even under mild conditions to thereby yield corresponding phthalaldehyde. After the completion of the reaction, a reaction product can be separated and purified by a separation means such as filtration, concentration, distillation, extraction, crystallization, recrystallization, and column chromatography or any combination of these separation means.
Industrial Applicability
The process of the present invention can efficiently produce phthalaldehyde from easily available and easily handleable raw materials without the use of halogen-atom-containing compounds and other hardly handleable compounds and metallic catalysts. Accordingly, the present invention is excellent both in economical efficiency and workability and is suitable as a process for the industrial production of phthalaldehyde.